Digital Radiography (DR) detectors directly transform received exposure energy to digital image data. These detectors commonly contain an array of light sensitive picture elements, or pixels, arranged in a matrix of rows and columns and a scintillator, consisting of a material, such as gadolinium oxisulfide, Gd2O2S:Tb (GOS) or cesium iodide, that absorbs x-rays incident thereon and converts the x-ray energy to visible light photons. In some configurations, the scintillator is in direct contact with the light sensitive array. The array of light sensitive elements can be any type of solid state sensor, such as a flat panel detector, a charge-coupled device, or CMOS detector. The light sensitive material converts the incident light into electrical charge which is stored in the internal capacitance of each pixel. The magnitude of the stored electrical charge is related to the intensity of the excited light, which is, in turn, related to the intensity of the incident x-rays. The radiation image exposures captured on radiation-sensitive layers are converted, pixel by pixel, to electronic image data which is then stored in memory circuitry for subsequent read-out and display on suitable electronic image display devices.
Much like video sensors and other types of two-dimensional solid state image detectors, DR detectors include several thousands of picture elements, or pixels. Inevitably, some number of pixels are found to be defective. Provided that this number is relatively small, the defective pixels can be tolerated and their impact on image quality can be minimized, as described by R Padgett and C J Kotre in “Assessment of the effects of pixel loss on image quality in direct digital radiography”, Phys. Med. Biol. 49 977-986, 2004. Frequently, specifications exist for the maximum number of allowable defects and the largest size of allowable defect clusters to maintain the required image quality.
Compensation techniques such as defect mapping and corrective image processing allow the use of DR detectors having defective pixels, provided that such pixels can be detected and proper steps taken for correcting the image. Defect mapping for image sensors is generally taught, for example, in U.S. Pat. No. 5,657,400 by Granfors et al. entitled “Automatic Identification and Correction of Bad Pixels in a Large Area Solid State X-ray Detector” and in U.S. Pat. No. 6,747,697 by Lin et al. entitled “Method and Apparatus for Digital Image Defect Correction and Noise Filtering”.
Defect mapping and correction procedures are commonly coupled with gain and offset calibration and correction procedures, which compensate for pixel-to-pixel variations in sensitivity and dark current. The most basic calibration and correction algorithms generally include two steps as taught by James A. Seibert, John M. Boone, and Karen K. Lindfors in “Flat-field correction technique for digital detectors,” Proc. SPIE Vol. 3336, 1998, p. 348-354. First, the dark signal of the detector (that is, the signal in the absence of any X-ray exposure) is obtained. Pixel by pixel variations in the dark signal of the detector are characterized to form a dark or offset map containing the dark variations. The offset map is then subtracted from the X-ray exposure in a process termed dark or offset correction. Secondly, variations in the sensitivity of the pixels are characterized. This is done by capturing one or more flat field exposures, which are then offset-corrected. The resulting image is the gain map. In the gain correction step, the offset-corrected X-ray exposure is divided by the gain map. Ideally, this two-step procedure compensates for any fixed pattern noise introduced by the detector.
Defect identification methods often explore anomalies in the gain and offset maps produced during calibration, for example by identifying pixels with gain and offset values that differ significantly from their surroundings, and by setting upper and lower thresholds for allowable values in gain and offset maps, to update the defect maps for a given detector as described in the previously cited Granfors et al. '400 disclosure and in U.S. Pat. No. 6,919,568 to Odogba et al. entitled “Method and Apparatus for Identifying Composite Defective Pixel Map”. Thus, periodic recalibration can help to manage defective pixels with conventional DR detectors and can help to produce corrected images with few, if any, visible defective pixels.
Conventional DR detectors generally accumulate few additional defective pixels over time and require infrequent recalibration. These detectors are often permanently mounted on a wall stand, in an examination table or some type of gantry or other type of adjustable framework that provides a secure mechanical mount for positioning the detector behind the patient and at a proper disposition with respect to the x-ray source. In addition to this mechanical support, the conventional DR system provides a “tethered” arrangement, with cabling for power and data to the DR detector. Even some tethered detectors may be somewhat portable; however, such devices typically have thick, rigid covers protecting the sensor and scintillating screen from any outside forces.
Advances in miniaturization, packaging, and data communications now make it possible to provide a more portable DR detector that may be as thin as a conventional film X-ray cassette. Wireless operation, moreover, eliminates the need for data cabling to the DR detector, making it easier to position the detector relative to the patient or other imaged subject. The use of an on-hoard battery eliminates the need for external power connection, enabling the DR detector to be positioned and handled in a manner similar to that of a film cassette or Computed Radiography (CR) cassette.
With the advent of more portable DR detector devices comes considerable promise for more flexible and adaptable imaging systems that can help to improve the efficiency and quality of patient care. However, there are some disadvantages related to the portability of such a device. Unlike film and CR cassettes, a portable DR detector contains a considerable amount of complex miniaturized circuitry. Rough handling of such a device, for example, can lead to some abrasion damage across the sensor surface, thus increasing the likelihood of defects and requiring additional calibration cycles in order to update the defect map. Moreover, with increasingly more compact packaging, environmental factors such as temperature variation can also cause the detector to need more frequent calibration. In addition, normal and rough handling of the detector can result in subtle motion of the scintillating screen relative to the sensor panel, resulting in localized changes in gain. These gain changes, which can be corrected by performing a gain calibration, have the appearance of misregistration artifacts.
Because of the factors discussed above, the required intervals between calibration procedures, needed for maintaining suitable image quality, are less predictable for fully portable detectors. One solution would be simply to require more frequent calibration for these units. Calibration could thus be required, for example, after a certain number of images were taken. However, this type of arbitrary interval negatively impacts productivity. Calibration procedures require radiology staff time and attention and each calibration reduces the overall utilization time of the DR detector.
Clearly, there is a need to monitor the calibration state of the detector'during regular clinical operation and to alert the user when calibration is needed. Various methods have been proposed for performing such monitoring. One solution, such as that proposed in U.S. Pat. No. 7,026,608 to Hirai, entitled “Gain Correction of Image Signal and Calibration for Gain Correction”, analyzes clinical images themselves to determine if recalibration is needed. If the threshold for recalibration is exceeded, the user is prompted to capture a flat field exposure after obtaining the clinical image. The ratio of the existing gain map to the newly acquired flat field is used to correct the image. This procedure may detect the need for calibration, but effectively disrupts operator workflow and increases access time for obtaining the current fully corrected clinical image. This disruption and time loss may be unacceptable in many clinical environments. In critical situations, such as in the emergency room or intensive care unit, for example, valuable time would be lost.
Another method for monitoring the calibration state of the detector during clinical operation is that described in U.S. Patent Application Publication Number 2007/0165934 entitled “Device and Method for Correcting Defects in X-Ray Images”, to Maac et al. In the method described by Maac et al., clinical images that have been fully corrected for gain, for offset, and for previously identified defects are routinely analyzed for new defects, and a defect candidate map is formed containing the new defects. Over time, the new defects from the defect candidate map are added to the permanent defect map only if they occur in a sufficient number of images. This method, although it may prove successful enough for a non-portable, mounted DR detector, falls short of what is needed for fully portable digital X-ray detectors. Such methods fail to distinguish between misregistration artifacts, which can be eliminated by performing a gain calibration, and truly defective pixels that need to be added to the defect map. For portable detectors, the method described in the Maac '5934 disclosure, despite the safeguards provided by adding defects to the candidate defect map first, would lead to the identification of pixels that do not belong in the permanent defect map. As a result, the detector would eventually exceed the threshold for the number of allowable defects and would have to be taken out of operation.
In summary, while there are some indications that conventional pixel defect detection methods may perform well enough when used within more permanent DR detector installations, these same methods do not appear to successfully address particular requirements of the portable DR detector. It has been found, for example, that conventional methods fail to distinguish between correctable misregistration problems that can be characteristic of portable devices, and truly defective pixels. This shortcoming limits the effectiveness of conventional approaches and makes these known solutions less desirable for the more rigorous requirements of the portable DR detector. There is, thus, a need for a method for monitoring performance of a portable DR detector in order to identify the need for recalibration, wherein this method is particularly suited to the needs of portable DR detectors.